In the case of an incomplete combustion of the air/fuel mixture in a gasoline engine, a plurality of combustion products, among which hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx) are subject to statutory limitations, are discharged in addition to nitrogen (N2), carbon dioxide (CO2), and water (H2O). According to the hitherto known related art, applicable exhaust gas limiting values for motor vehicles can only be adhered to if a catalytic converter is used for exhaust aftertreatment. By using a three-way catalytic converter, it is possible to convert the above-named emission components.
A concurrently high conversion rate for HC, CO, and NOx is achieved in three-way catalytic converters only within a narrow lambda range around the stoichiometric operating point (lambda=1), the so-called “catalytic converter window.”
In order to operate the catalytic converter within this lambda range, a lambda control, which is based on the signals from lambda sensors downstream and upstream from the catalytic converter, is typically employed in present engine control systems. To control the lambda upstream from the catalytic converter, the oxygen content of the exhaust gas upstream from the catalytic converter is measured using the lambda sensor. The control corrects the fuel quantity from the pilot control as a function of this measured value. For a more accurate control, the exhaust gas downstream from the catalytic converter is additionally analyzed using an additional lambda sensor. This signal is used for a setpoint control which is superimposed on the lambda control upstream from the catalytic converter. A jump lambda sensor which has a very steep characteristic at lambda=1 and can therefore indicate lambda=1 very accurately is generally used as the lambda sensor downstream from the catalytic converter.
In the control concepts, the lambda sensors can be analyzed with regard to dynamic errors and corrected, if necessary. Methods for diagnosing the dynamics of lambda sensors are provided in DE 10 2014 209 392 A1, DE 10 2012 209 195 A1, DE 10 2009 045 376 A1, and DE 10 2011 081 894 A1, for example.
Present control concepts have the disadvantage that they detect an exit from the catalytic converter window only at a late point based on the voltage of the jump lambda sensor downstream from the catalytic converter.